I must have had too much time on my hands, because I recently decided to do some fine tuning of my espresso machine. I insulated the boiler, reduced the boiler temp... what next? Well, adjust the brew pressure of course! But my Rancilio L7 does not have a pump pressure gauge, only a boiler pressure gauge. So I stopped by a local hardware store and walked out with a pressure gauge and reducing coupler.

This is old hat to some folks (e.g., see Dan Kehn's post on this thread), but maybe it'll be of interest to others. I chose a pressure gauge from the plumbing section with a 2" dial and 1/4" NPT diameter thread on a lower mount, rated to 200 psi (14 bar). I used a 3/8" to 1/4" female brass reducing coupler to attach it to the portafilter, with a couple twists of Teflon tape to make it watertight.

Uh, no. You have to unscrew the gauge (at least) before brewing. This is for setting the brew pressure, not monitoring it during the pour.

Actually, that brings up an interesting point. The measured pressure may be slightly higher using a PF gauge, since there is no liquid pushing out through the coffee grinds. It should give you the maximum pressure setting - anything more will cause pressure to be released through the overpressure/relief valve. I assume it's useful nonetheless (since you can purchase commercial versions).

Has anyone checked for differences between brew pressure gauge readings during a shot, and pressure measurements from a PF gauge such as this one?

This gauge is dry - largely because that was all I found in the hardware store :-). My machine has a rotary pump, and I have not seen any pressure fluctuations. Just a slow ramp up to the current pressure setting of 9 bars, then rock steady. You might consider a glycerin-filled gauge or a snubber if you get pressure oscillations (more likely with a vibe pump).

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